Noctilucent Cloud Page

Updated
24th June 2018

During those short Summer Nights when the sky never really
gets dark Noctilucent Clouds (NLC) offer Astronomers something different to observe.
While they are a bit closer to Earth than our
normal objects ( with the exceptions of Meteors), surprisingly little is known
about these clouds that form in the upper reaches of the atmosphere. Part of
the reason that so little is known is that they occur at a height of typically
70 - 90 kms. This is a very difficult region to reach as it is too high for aircraft
and balloons and too low for spacecraft to operate. As a result the few measurements
have been of a transient nature as spacecraft pass through this height on the
way to orbit. These suffer from the problem of contamination by the rocket exhaust
as one (in)-famous experiment on the Space Shuttle found.

These clouds are only visible for a few weeks each side
of mid-summers day. To observe them you need to wait till the Sun is at least
6 degrees below the horizon so that the sky is dark enough and so that lower
Cirrus clouds are not illuminated by the Sun.

One of the brighter displays on the 14th July 2006 is
shown in the images on this page. They were photographed from the grounds of The
Royal Russell School . Earlier at the beginning of the meeting I had showed
some pictures showing what I initially thought were Noctilucent Clouds earlier
that week - I realised my mistake when I was imaging them and saw an Aircraft
creating another one - they were just high aircraft condensation (con) trails.
Until you have actually seen a NLC it is an easy mistake to make but the NLC
when I saw it was much lighter blue and has a luminous look against the Sky
Background. This display was widely see from the Southern UK and lasted about
an hour. I was very lucky to have a digital camera with me - I did not have
a tripod and had to use a convenient fence post to try to steady the camera.
Motto - carry a camera and miniature tripod !

A search on Google will find a number of number of sites
on observing NLC's. A couple of the ones that I recommend are The Noctilucent Cloud Observers Home Page in
addition to information on observing and recording image NLCs there is a page
that shows the latest sightings. This page also shows images of the NLCs. However
you should be aware that images appear on this page of cloud formations that
do not appear to be NLCs so you need to be careful if using these for confirmation
of your sighting.

The
International Association of Geomagnetism and Aeronomy NLC
web site has a PDF copy of a guide to
observing NLCs. This contains a lot of interesting and useful information.
Unfortunately the link on the page that should get you to the images does not
work but the images in the PDF file are of reasonable quality.

Some more of my images of NLCs
taken on the 14th July 2006 can be found here.
Examination of the images will show some evidence of camera shake - hence my
recommendation to carry a tripod. At the time the first image was taken at 22:15
BST the Sun was about 8 degrees below the horizon from Croydon. The last was
taken at about 22:30 BST when the Sun was about 9.5 degrees below the horizon.
By the time I had driven home at around 22:50 when the Sun was just over 11
degrees below the horizon there was very little visible.

There
are some good movies of NLC taken by Jacek
Stegman of MISU .
These are taken with a standard digital camera fixed to a North facing window.
A computer is used to take the time lapse images and store the result.
Unfortunately the movies are no longer available from his web site but one is
available on UTube
here.

One useful tip from the PDF guide
above is that the light from NLCs tends to be quite strongly polarised. They
recommend using a polarizing filter to aid visual confirmation - it is not clear
if the use of a polarising filter will help imaging - I am waiting for more
NLCs to find out.

It is easy to confuse high cirrus
- either natural or from aircraft con trails with NLC. I have created a page
with some images of False NLC here
to aid you spotting the real thing..

To observe NLCs the sun has to
be between 6 and 16 degrees below the horizon. At less than 6 degrees the sky
is too bright and cirrus clouds as well as the NLC will be illuminated. When
the Sun gets below 16 degrees the Sunlight does not illuminate the NLCs. It
should be noted that as the Sun gets lower the point that the NLCs are illuminated
moves towards the horizon in the direction of the Sun.

To aid observations I have use the
spreadsheet described below. At present it is set for London UK but you can
easily change this to your location. The number in the table shows how far the
sun is below the horizon.. The numbers in the table are shown in three colours - black for when
the Sun is too high or low, Dark Blue for when the Sun is between -6 & -12
degrees - this offers the best chance of observing the clouds. Between 12 &
16 degrees the numbers are in pale blue, the NLC will be much lower in the Sky
and you need a clear NW to NE horizon to see them. The spreadsheet is available
on the link below:

If you live or are observing away from London the
US Naval Observatory have a calculator here that will show you the Sun's altitude but unfortunately
only calculates the altitude to -12 degrees.

I have revised my
calculation
spreadsheet so you can now enter your location and time zone
and the table will automatically update. The
year automatically. Please
note that to get the calculations to work you need to enable the macros
in this spreadsheet - otherwise the cells will be filled with a #NAME? error.
The only note of caution is that
the calculation allows for atmospheric refraction as do the calculations
above. I need to investigate what values are used for refraction when the
Sun is below the Horizon ( it's about 0.8 degrees at the Horizon. This leads to the interesting question 'Do the quoted values for the amount
the Sun has to be below the Horizon allow for refraction ?' If you know
the answer please contact me. V4 uses conditional
formatting to highlight the times for today's date in yellow making this
easier to see. Unfortunately this overrides the blue formatting of the
angles when NLC are likley to be seen but they are still shown in bold and
are fairly obvious from the blue text in the adjacent rows.

An
additional spreadsheet (NLC-Now) is available here
( Excel 2007 & later)
that shows the Sun's altitude and azimuth in real time ( derived from your
PC Clock). You need to enable
the macros for the spreadsheet to work. The accuracy is dependent on your
PC Clock. To use the spreadsheet you need to enter your observing location
as well as the time zone and daylight saving time. The colour of the displayed
Sun's altitude changes when the Sun is at the best height for NLC to be
observed.

The time when the Sun is between 6 and 12 degrees
below the horizon is known as Nautical Twilight and can be calculated using some
astronomy planetarium programmes. When the Sun is between 12 and 18 degrees
below the horizon this is Astronomical Twilight - at the end of Astronomical
Twilight the Sun is 2 degrees lower than the figure quoted for the end of visibility
of Noctilucent Clouds. As a result we can use the times for Nautical Twilight
for the times for the best opportunity but have to be aware that the NLC are
not likely to be visible near the end of Astronomical Twilight.

Two calculators for the times of Twilight can
be found here
from spectralcalc.com (use the Solar Calculator Option) and from the US Naval
Observatory here
select Nautical or Astronomical twilight from the 'Type of Table'.

Observations in the southern hemisphere summer are more
difficult as there is little inhabited land far enough South to view the NLC.

Some Tips:

Carry a camera at all times. Modern digital cameras
are quite capable of recording images that need exposures of a few seconds.

Know how to use the camera in low light - to set
the focus to infinity and turn the flash off - often the 'Landscape' mode
is a good start. Also you might need to increase the ISO rating if this
is not automatic.

Practice on aircraft con trails after sunset - there
are plenty to image !

The NLC will be in the direction of the Sun i.e.
North West after sunset to North East before sunrise ( in the northern
hemisphere)

They may be visible in gaps in lower (dark) cirrus
cloud)

Carry a small tripod - it is impossible to hand hold
the camera.

Carry a polarising filter to aid identification

If possible have a polarising filter that attaches
to your camera - difficult with most point and shoot cameras

The sky needs to be clear for in the direction of
the NLC - even over the horizon - a bank of thick cloud will stop the Sunlight
as it skims the Earths surface. This condition may be achieved by a 'High
Pressure' area to the North of your position but beware that Summer Highs
can easily become polluted and not very transparent. Keep looking at the
weather charts and Satellite Images. More information of sources of Weather
Information can be found on my Weather
Page.

Look out every clear night and morning at the times
shown in Dark Blue in the spreadsheet

Beware that bright moonlight can illuminate high
cirrus clouds even when the Sun is well below the horizon - these clouds
will not be as blue as the true NLC.

Low powered binoculars with a wide field of view
and a large exit pupil may be useful

Beware that
moonlight can illuminate high normal clouds and aircraft con trails so they
look a bit like Noctilucent Clouds. Aircraft con trails are much lower than
NLCs.

Keep a record
of your observations for Morning and Evening for every day during the season.
It is important to know if there were clouds, if you were not able to observe,
if it was clear and there was no NLC or it was clear and you observed NLC

NLC
Predictions

It
is very difficult to predict NLC but the Sky to the North of your location needs
to be clear not only so you can see the clouds but so the Sun can shine onto
the clouds. The normal weather satellite images help but there is not much information
on how far north the sky has to be clear to allow the Sun to shine on the NLC
due to the amount of refraction in the atmosphere.

In
addition there has to be sufficient moisture in the Mesosphere and the temperature
has to fall low enough ( The Mesosphere temperature falls in the Summer). A
plot of the current Mesosphere temperatures can be found on this
link ( Note this
page does not refresh itself so you may need to reload the page to ensure you
have the latest data). There is no exact threshold when NLC form but it provides
some clue.

AIM Mission:

On the 25th April 2007 NASA
launched the AIM ( Aeronomy
of Ice
in the Mesosphere) mission specifically to study Noctilucent
Clouds, they have published some images from space but 2007 was a poor season
at least from the UK. There are
two sites that contain information from the mission - the NASA summary site
here
and a more detailed page from the mission managers at Hampton University (USA)
located here.

Details of the CIPS imager on
the AIM spacecraft are here
look also at the highlights and the data page to see what the NLCs you spotted
look like from space !

The AIM CIPS 'Daily Daisy' showing the
location of NLC is shown on the left side of the
spaceweather.com homepage. This shows the
Southern Hemisphere during winter and is changed to the Northern Hemisphere
during May.

May 2017
Announcement from AIM:

Spaceweather.com carried the following
announcement from AIM on 16th May 2017

"Currently, we are not retrieving cloud
data. The orbit of the spacecraft has precessed, and we are transitioning to a
new way of pointing. We need a bit more time to settle the spacecraft pointing
and hope to be back online by the end of May."

Hopefully this will get sorted soon so we
can get a better idea of when NLC are far enough south to observe. Until then it
is back to the Mark One eyeball !

Weather
Charts:

One
of the things that need investigating is what were the meteorological conditions
when NLC was sighted. One useful thing to look at is the Surface Pressure
Analysis or Synoptic Chart. An archive of these is available here.
Enter the date of the chart you require in the box in the bottom left, click
the 'los!' button and you will get the 00:00 UTC chart. You can then select
the other charts for that day by using the arrows on the bottom right. Just
be careful with the date of the 00:00 UTC chart - this is the first for the
new day not the last of the previous day.

How far away are NLC ?
added 12th June 2017

To observe the NLC the path between the
observer and the NLC needs to be clear of other low level clouds. The table
below shows the calculated distance to NLC for different observation angles,
assuming a height of 80km for the NLC. Since the path is diagonal through the
atmosphere the line of sight can pass above distant low level clouds. The
distances calculated probably under estimate the distances at low elevations as
the calculation does not include refraction. At high elevations the height of
the cloud at 80 km / 50 miles causes a significant error in the actual
distance as measured on the surface.

I
use a simple spreadsheet to record the results for each morning and evening
during the Season from May till August. The
template has been modified so it records the Sun's altitude at the start &
end of each observing session. This also has a link to a plot of Mesosphere Temperatures
and Water Vapour Levels. A copy of the latest version updated in 2011 Excel
Template can be downloaded here.
You need to enter your name, observing location and latitude and longitude.
Use the F9 (recalculate) key if you want to update the time and altitude calculation.

The
spreadsheet is intended to show when NLC was NOT observed as well as the rare
occasions it was. The entry 'NO' for no observation means just that - for one
reason or another you were not able to make an observation. If it was cloudy
use C1 to C8 with your judgment of how many 8ths of the sky was covered. I usually
just record the NW to NE sector as that is the only place that NLC will be seen.
If you were observing and no NLC were seen put an X in the NLC column, this
applies if it is clear or partially cloudy - if you think you would have seen
them if they existed put an X in the column.. If you observed NLC put 'NLC'
in that column. Times are in UTC - the clock at the top is just to remind you
what the UTC time is if you are on BST. Observers in other time zones will have to edit
the formula for the appropriate offset. You need to create a new spreadsheet
for each month and set the top left date cell to the first of the month &
correct year - the
rest should then fill in automatically.

Remote
Control on Canon 10D + series cameras

To capture images of NLC it is desirable
to leave your camera imaging for a couple of hours post Sunset and another couple
of hours pre dawn. I have tried various software to control my camera but none
is really satisfactory. The ideal is to have intervals of a minute or so and
also to allow for mirror lockup which requires a double press of the shutter
button ( once to lock the mirror up) and a second followed after a short interval
to take the image.

One piece of software that will do this is
IRIS but as with all the software for the earlier Canon 10D to 40 D cameras
it needs an interface and special connection to the remote shutter connection
to work. This
presentation (pdf) shows the circuit I have used and also how it is constructed.
This can be used for astro imaging as well.

Having
constructed the interface and run some trials with IRIS I have identified a
problem in that IRIS only allows you to set the interval from the end of one
shot to the start of the next. As a result if you are using auto exposure as
in the settings below the gaps between Frames increases. Perfectly acceptable
for astronomical imaging but in my case I want to convert the still images into
a movie. The Increase in intervals would result in the star motion and any NLC
speeding up as the exposure time increases. It is possible to use IRIS but you
need to ensure the sum of the mirror lockup time, the exposure time and the
card write time adds up to the frame interval you desire.

As
a result of this I have gone back to my original plan which is to use a Raspberry Pi
to control the Camera via a PiFace interface. Originally the RaspberryPi would
freeze when it lost the network connection when it was turned off at night. However
updating the RaspberryPi using 'sudo apt-get update' followed by 'sudo apt-get
upgrade' appears to have solved this problem. I have left my Pi running for
several hours without any more lockup problems.

The
camera is controlled via a very simple Python Programme. The only thing to be
careful of is that the Raspberry Pi has a network connection when it starts up. If not
the Pi's time will be set to the time it was last shut down. This does not cause
problems at present but a future development is to start and stop the image
sequence at specified times. If you don't have a network connection it is
possible to set the time using a GPS module, this needs some editing of the
Raspberry Pi configuration software. It is probably best to search on the web
for the latest version of how to do this.

Since the sky brightness will change a lot
during the hours post / pre twilight the camera exposure will change. I use
the camera auto exposure system to adjust for this which it seems to perform
quite well. The camera settings I use are:

Revised 24th June 2018

ISO 400 or 800 ( determined by the need to have
the exposure shorter than the time interval (minus the mirror lockup time)
when the sky is darkest. This will depend on how fast your lens is as
well. Adjust the ISO speed and the lens aperture to make sure the
exposure can complete within the allocated time when the sky is at its
darkest. My Python Intervalometer allows a maximum exposure of about 20
seconds. ISO 400 - 800 usually produces the highest dynamic range though
it varies between camera makes and models.

Picture quality - Large JPG or (RAW & JPEG if
you have enough storage space and download time). RAW increases the
dynamic range and allows you to alter the colour temperature in post
exposure processing.

Mode - Av with lens aperture set wide open ( f2.8
or faster lens is best)

Power - AC adaptor -
if you don't have an adaptor use a newly charged battery each night.

White Balance - After testing different
settings on the extensive display on the morning of the 23rd June 2018 I
have come to the conclusion that 'Daylight' (5700k) produces the better
results than Auto White Balance on my Canon Cameras. The best option is
probably to take JPGs with Daylight Balance and also take RAW images,
that way you can alter the colour temperature in your image processing
software. With Canon Cameras this is Digital Photo Professional

Auto Power down - off

Mirror lockup - on

Image review - 2 secs to get confidence the images
have been taken. Not advisable to leave it on continuous as the
backlight heats the CCD / CMOS sensor and increases the noise.

Focus - Manual (AF will fail when it gets dark
or may focus on objects closer than infinity). If no
focus is obtained no image will be taken. Better to manually focus on
infinity and tape the focus and zoom rings if required.